This invention relates to an automobile internal combustion engines and particularly, to electric spark-plugs for the automobile internal combustion engines. An object of this invention is to provide an antipollution internal combustion engine which is superior in exhaust gas properties.
The internal combustion engine provides power through the burning operation of gas mixture between air and hydrocarbon fuels such as gasoline, petroleum gas, etc. As shown in FIG. 1, the respective concentrations of exhaust gas constituents such as nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC), etc. which are discharged from the internal combustion engine, vary in accordance with an excess air ratio F (which means a ratio of an air fuel ratio to an equivalent air fuel ratio, the air fuel ratio indicating a ratio of air mass and fuel mass, the equivalent air fuel ratio indicating an air fuel ratio provided that H.sub.2 O and CO.sub.2 have been produced stoichiometrically through reaction of the fuel and the oxygen. Accordingly, the smaller the value of the excess air ratio F is, the higher the fuel concentration is. The greater the value of the excess air ratio F is, then the lower the fuel concentration is). For example, the gas mixture in the range of 0.9 &lt;F &lt;1.25 produces a large quantity of NOx due to high combustion temperatures (approximately 2,000.degree. K to 3,000.degree. K). The gas mixture in the range of F &lt;0.9 produces relatively small quantity of NO.sub.x, but an extremely large amount of CO and HC. Accordingly, in order to improve the exhaust gas properties to develop antipollution internal combustion engines, it is required to burn lean gas mixture in the range of F &gt;1.25, or to effect exhaust gas recirculation (EGR) to make fuel concentration lean to lower the combustion temperature of the gas mixture down to approximately 1,500.degree. K or less.
In the conventional thin electrode type spark-plug, for example, a spark-plug shown in FIG. 5 and by Table 1, line W, which comprises a grounded electrode 1 composed of wide and long plate and with a high-tension electrode 2 of thin cylinder with flat end face, has, as shown in FIG. 42, extremely narrow ignitable region (a region below the curve). FIG. 42 shows the relationship of the ignition-limit excess air ratio F.sub.L of the gas mixture under 1 atmospheric pressure and room temperature vs. the electrode gap distance, namely, spark gap L.sub.s. As seen from FIG. 42, in the conventional spark-plug, the ignition of the lean gas mixture in the range of the F &gt;1.25 can be realized only when the spark gap is greater than L.sub.s * = 2.28m ( the L.sub.s * shows a measured ignitable spark gap for an excess air ratio of ignition-limit of F.sub.L = 1.25).
Table 1 ______________________________________ (Prior art) Grounded High-Tension Marks of F.sub.L -L.sub.s Types of Electrode Electrode Characteristic Elec- Dimensions Dimension L.sub.s * Curves trodes (mm) (mm) (mm) ______________________________________ W Fig. 5 2.7 width 1.0 diameter 2.28 1.3 thickness 5 length W' Fig. 6 2.7 width 1.0 diameter 2.04 1.3 thickness 5 length ______________________________________
Also, as in the other conventional spark-plug of FIG. 6 and Table 1, line W', according to a thin electrode type spark-plug which comprises the same high-tension electrode 2 as that of FIG. 5, and a grounded electrode 1 with U-shaped groove 1' along the lengthwise direction of the discharging plane on the wide and long plate, the characteristic curves between the excess air ratio F.sub.L of ignition-limit and the electrode gap distance L.sub.s are improved as compared with the curve W, as apparent from the curve W' of FIG. 42. However, the ignitable region is so narrow that the ignition of the lean gas mixture in the range of the F &gt;1.25 can be realized only when the electrode gap distance is greater than L.sub.s * = 2.04 mm.
In using an ignition power supply on the market, dischargeable-limit gap distance Ls for discharge with two-electrode type plug is about 2 mm for a gas mixture (gas mixture on heavy loading of the internal combustion engine with a compression ratio of approximately 10), which is compressed into molar density as eight times high as that under one atmospheric pressure. Accordingly, in the conventional spark-plugs, it has been difficult to directly ignite the lean gas mixture where F &gt;1.25. In order to eliminate such difficulty, the following three methods have been proposed.
A torch igniting means has been considered as a first method, according to which an antipollution internal combustion engine is realized by use of spark-plugs of the prior art. The torch makes it possible to ignite such lean gas mixture that has an air fuel ratio at the explosion limit. Accordingly, by making the gas mixture only near the spark-plug high in fuel concentration to effect the igniting operation, the excessively lean gas mixture (on the whole) can be burned through the formation of the torch. However, the disadvantages with the above torch ignition means are that combustion subchamber, and subcarburetor, or extra fuel injected system, etc. are required to realize the above-described burning operation of the lean gas mixture.
Secondly, it is also possible to take measures for individual operation modes. During starting and warming-up operations, idling (no-load) operation and engine-braking (negative load operation), less NO.sub.x are produced independently of the value of the excess air ratio F, because of the low temperature and low pressure of the gas mixture. However, in such modes an F valve of nearly equal to 1 or smaller is required to assure igniting operation. Thus, the number of revolutions of the engine in idling mode has to be high to ensure F .apprxeq. 1, and hence, the fuel consumption increases adversely. In the heavy loading operation mode and the others than the above mentioned modes, temperature and pressure of the gas mixture is high, and therefore, the lean gas mixture in the range of approximately F = 1.2 or smaller can be spark-ignited. Accordingly, the less NO.sub.x is produced with F .apprxeq. 1.2 and simultaneously better heat transfer rate through the cylinder wall of the engine. The disadvantage of such individual-operation-mode measure is that the internal combustion engines should provide a system that the value of the excess air ratio F is to be adjusted and controlled in the extremely narrow allowable value range for every operation mode.
Thirdly, it is possible to provide an internal combustion engine wherein an excessively rich gas mixture (on the whole) is ignited to burn at relatively low temperature for preventing the NO.sub.x production and CO and HC are rear-treated through the use of catalyst or thermal reactor. However, in order to realize the above requirements, rear-treating system including catalyst, thermal reactor, air pump, catalyst antioverheat equipment, etc. are required, thus resulting in power loss in exhaust system increased fuel consumption due to excessively rich fuel, sulfuric acid mist and scattering heavy metal.
The internal combustion engines used in the abovementioned first, second and third methods have such disadvantages as increased weight and cost of the engines, complicated adjustment of the optimum operating conditions of the engines, inferior stability of the optimum operation, difficulyt mass production controlling, and complicated, difficult adjustment of the mammeo us use.